1. Field of the Invention
The present invention relates to nonreciprocal circuit devices. In particular, the present invention relates to a nonreciprocal circuit device, such as an isolator and a circulator, preferably for use in a microwave band, and also relates to a manufacturing method of the nonreciprocal circuit device.
2. Description of the Related Art
Conventionally, nonreciprocal circuit devices, such as isolators and circulators, have a characteristic that allows signals to be transmitted only in a predetermined specific direction but not in the opposite direction. By making use of such characteristics, isolators, for example, are used in transmission circuits of mobile communication apparatus, such as automobile phones and portable phones.
In this type of nonreciprocal circuit device, to protect an assembly of a ferrite having a plurality of central electrodes and permanent magnets applying a direct-current magnetic field to the ferrite from an external magnetic field, the assembly is surrounded by a ring-shaped yoke (see, WO 2006/011383) or by a box-shaped yoke (see, Japanese Unexamined Patent Application Publication No. 2002-198707).
However, in a conventional nonreciprocal circuit device, a yoke obtained by processing a soft iron or other suitable material into a ring shape or a box-shaped yoke is used as a magnetic shield. Consequently, a significant amount labor and cost are required for the processing and assembly of the nonreciprocal circuit device.